Reducing the capacitor voltage ripples of the half-bridge sub-modules (HBSM) and full-bridge sub-modules (FBSM) in a hybrid modular multilevel converter (MMC) is expected to reduce the capacitance, volume and costs. To address this issue, this paper proposes a dual harmonic injection method which injects the second harmonic circulating current and third order harmonic voltage into the conventional MMC control. Firstly, the mathematical model of the proposed control is established and analyzed. Then, the general strategy of determining the amplitude and phase angle of each injection component is proposed to suppress the fluctuations of the fundamental and double frequency instantaneous power. The proposed strategy can achieve the optimal power fluctuation suppression under various operating conditions, which also has the advantage of reducing the voltage fluctuation difference between HB and FB SMs. The correctness and effectiveness of the proposed strategy are verified in simulations in PSCAD/EMTDC. Index Terms-Hybrid modular multilevel converter (MMC); second harmonic circulating current; third harmonic voltage; dual harmonic injection; fluctuation suppression. NOMENCLATURE Symbols: Descriptions Udc Pole-to-pole dc voltage. Uac Amplitude of the AC-side phase voltage. Im Amplitude of the AC-side phase current. ω Fundamental angular frequency.

1. O p ti m al s e c o n d-h a r m o ni c c u r r e n t inj e c tio n fo r c a p a ci to r volt a g e fluc t u a tio n r e d u c tio n in h y b ri d M M C s u n d e r g ri d-si d e SLG fa ul t s. IEE E Tr a n s a c tio n s o n Po w e r D eliv e ry file

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